Permit requirements are local and the shape of them is consistent almost everywhere.
Usually no permit
Painting. Replacing a faucet, a toilet, a light fixture or a vanity in the same position. New accessories. A new mirror.
Like-for-like replacement that does not alter plumbing, electrical or structure is generally maintenance.
Usually a permit
Moving any plumbing fixture. New drain, new vent, new supply routing.
Electrical changes — a new circuit, moved or added receptacles, a new fan circuit.
Structural work — removing or altering a wall, changing an opening.
A new bathroom where there was not one.
Replacing a tub or shower in many jurisdictions, because it involves plumbing connections and waterproofing.
The rule of thumb that holds: if the walls open, ask.
The two inspections
Rough-in, after the plumbing, electrical and framing are done and before anything is covered. This is the one that matters — it is the only moment anyone will ever see the pipes, the wiring, the venting and the blocking.
Work cannot be covered until it passes, so a failed rough inspection stops the job rather than slowing it.
Final, after everything is complete and the fixtures are set.
Some jurisdictions add a waterproofing or a framing inspection between them.
Why it is worth having rather than avoiding
The obvious argument is legal. The practical ones are better.
An independent set of eyes on the buried work. The inspector is not paid by the contractor and is looking specifically at the things that are about to become invisible.
It creates a record. Permitted work appears in the property file, which matters at sale.
Unpermitted work surfaces at sale. A buyer's inspector notices a bathroom that is newer than the permit history, and the transaction slows or the work has to be opened for retrospective inspection.
Insurance. A claim arising from unpermitted work can be contested.
Who pulls it
Usually the contractor, and it should be stated in the contract who does.
A homeowner pulling the permit on work a contractor performs takes on the responsibility for compliance, which is not the arrangement most people intend. Where a contractor asks the homeowner to pull the permit, that is worth understanding rather than agreeing to reflexively.
A contractor who suggests skipping the permit is proposing that you carry a risk so their schedule is simpler. That is worth knowing about them regardless of what you decide.
The schedule effect
Permits take days to weeks to issue depending on the jurisdiction, and inspections have to be booked.
That is real time in the schedule and it belongs in the plan rather than as a surprise. Contractors who work in an area regularly know the lead times.
What to keep afterwards
The permit, the inspection sign-offs, and photographs of the rough-in before it was covered.
That last one is not required by anybody and it is the most useful document in the folder ten years later, when someone needs to know where the pipe runs before drilling into a wall.
Cost, in perspective
Permit fees for a bathroom are typically a small fraction of the project cost — often a few hundred dollars depending on the jurisdiction and the valuation of the work.
Against that, an unpermitted bathroom discovered at sale can cost a great deal more, either in a price adjustment or in opening finished work for retrospective inspection.
The fee is not the reason people skip permits. The schedule is, and the schedule effect is measured in days rather than weeks in most places.
Historic districts and condominiums
Two situations with additional approvals beyond the building permit.
Historic districts may restrict what can change, particularly anything visible from outside, and approval can take considerably longer than a standard permit.
Condominiums and co-ops almost always require board approval, often with requirements about working hours, insurance, and protection of common areas. Several also require an architect's letter for anything involving plumbing.
Both should be checked before the design is finalised rather than after.